With the market demands for brightness, color saturation, service life, non-toxicity and environmental protection of the projection apparatus, the type of the light source applied on the projection apparatus is evolved from an ultra-high pressure mercury lamp (UHP lamp), a light emitting diode (LED) to a laser diode (LD).
Currently, the costs of high-brightness red laser diodes and green laser diodes are still high. To decrease the cost, a part of the blue light of a blue laser diode is usually applied to excite the phosphor powders on the phosphor wheel to generate a yellow light or a green light, and another part of the blue light of the blue laser diode is reflected by or penetrates through the phosphor wheel. A desired red light is filtered from the yellow light, and then combined with the reflected or penetrated blue light to constitute the three primary colors of red, green, and blue required for the projection images.
Conventionally, a heat conductive glue is applied to adhere a sheet-like phosphor layer made by sintering to a substrate to fabricate a phosphor wheel, or further arranging a reflective layer, such as a sintered reflective layer, between the sheet-like phosphor layer and the substrate. However, the heat conductivity (K) of the heat conductive glue is about 2 W/(m·K) [Watt/(meter·degree)] to 10 W/(m·K), and the heat conduction of the heat conductive glue decreases sharply with the increase in the thickness of the heat conductive glue. If the thickness control of the heat conductive glue is unstable, the heat conductions of the fluorescent layer and the reflective layer will be affected obviously; thereby affecting the wavelength conversion efficiency of the phosphor layer and the reflection efficiency of the reflective layer. Moreover, the material may be deteriorated and damaged (blackened) due to the excessive temperature of the phosphor layer and the reflective layer.
In addition, transparent silicon is also applied to replace the heat conductive glue conventionally. A reflective coating film is disposed on the substrate to replace the reflective layer, and the thickness of silicon is controlled to less than 0.05 mm. However, the heat conductivity of silicon is merely about 0.2 W/(m·K) to 0.3 W/(m·K). Therefore, even if the thickness of the silicon is very thin, the heat conduction of silicon is limited, and thereby the wavelength conversion efficiency of the phosphor layer is poor, and the material may be deteriorated or damaged.
The information disclosed in this “BACKGROUND OF THE INVENTION” section is only for enhancement understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Furthermore, the information disclosed in this “BACKGROUND OF THE INVENTION” section does not mean that one or more problems to be solved by one or more embodiments of the invention were acknowledged by a person of ordinary skill in the art